1. Field of the Invention
The present invention relates to a semiconductor rectifier and, more particularly, to a semiconductor rectifier suitable as a diode used in inverter circuits, chopper circuits and the like and a method of driving the same.
2. Discussion of the Background
FIG. 1 is a sectional view of a conventional diode having a high blocking voltage showing a configuration of the same. In this high blocking voltage diode, an n-type base layer 12 is formed on an n-type emitter layer 11, and a p-type emitter layer 13 is formed on the n-type base layer 12. An anode 14 is formed on the p-type emitter layer 13. A cathode 15 is formed on the backside of the n-type emitter layer 11.
In a state in which a positive bias relative to the cathode 15 is applied to the anode 14, i.e., in the so-called forward bias state, electrons are injected from the n-type emitter layer 11 into the n-type base layer 12 and holes are injected from the p-type emitter layer 13 into the n-type base layer 12 to put the high blocking voltage diode in a conducting state.
In the conducting state, since the injected electrons and holes are accumulated in the n-type base layer 12, the resistance of the n-type base layer 12 decreases, which also decreases the resistance of the device as a whole.
A description will now be made on a reverse recovery operation that occurs during transition from the conducting state to a blocking state. FIG. 2 is a circuit diagram of a DC chopper circuit illustrating the reverse recovery operation of a diode. Although circuits such as inverter circuits in practical applications are different from this circuit, they encounter the same phenomenon as that in this circuit during the reverse recovery operation. FIG. 3 is a schematic view of voltage and current waveforms of a diode during reverse recovery.
Referring to FIG. 2, the reverse recovery operation means an operation wherein when a switching device Dm is turned on while a forward current If is flowing through a diode Dd, a power supply voltage V is applied to the diode Dd in the reverse direction to cause an abrupt transition of the diode Dd from the conducting state to the blocking state.
As shown in FIG. 3, the switching device Dm is turned on at a point in time t1 at which a diode current Id decreases at a current transition rate di/dt based on the power supply voltage V and stray inductance Ls of the circuit and is increased to a maximum reverse current Irm at a point in time t2.
A depletion layer begins to spread at the junction of the n-type base layer 12 and the p-type emitter layer 13 at the point in time t2, which causes a diode voltage Vd to start to rise. At the same time, the diode current Id flowing in the reverse direction begins to decrease. Thereafter, the diode voltage Vd rises beyond the power supply voltage to reach a maximum value due to the presence of the stray inductance Ls of the circuit and then approaches the power supply voltage. At this time, a flow of a tail current is caused by residual carriers in the vicinity of the cathode.
In a diode having a conventional structure, a large amount of carriers must be accumulated in the n-type base layer 12 in order to reduce loss in the conducting state associated with resistance during conduction. This increases loss during reverse recovery which is given in terms of a value reached by obtaining time-integral of the product of the diode voltage Vd and the diode current Id during reverse recovery.
If the amount of carriers accumulated in the n-type base layer 12 is reduced to reduce the loss during reverse recovery, the loss in the conducting state is increased.
As described above, in a diode having a conventional structure, a design to reduce the loss during reverse recovery results in an increase of the loss in the conducting state, and a design to reduce the loss in the conducting state conversely results in an increase of the loss during reverse recovery. This type of loss characteristics are determined at the phase of designing a diode and are regarded uncontrollable during operation. Therefore, a conventional diode is designed by finding an optimum point on a tradeoff curve such that the sum of the loss during reverse recovery and the loss in the conducting state is minimized.
Although such an optimum design is effective when the ratio between the periods of the conducting and blocking states of the diode is fixed, it is ineffective when the ratio between the periods of the conducting and blocking states varies. The reason is that a change in the ratio between the periods of the conducting and blocking states results in a change in the optimum point on the tradeoff curve.
It is therefore difficult to reduce the sum of the loss during reverse recovery and the loss in the conducting state of a diode where the ratio between the periods of the conducting and blocking states of the diode varies every moment as in the case of a diode used in an inverter circuit, chopper circuit or the like.
As described above, a diode having a conventional structure has a problem in that an increase in the amount of carriers accumulated in the n-type base layer 12 to reduce the loss in the conducting state increases a reverse current that flows upon transition from a forward bias state to a reverse bias state, thereby increasing the loss during reverse recovery.
On the other hand, the problem of an increase of the loss in the conducting state arises when the amount of carriers accumulated in the n-type base layer 12 is reduced to reduce the reverse current in an attempt to reduce the loss during reverse recovery.
Further, even if an optimum design for minimizing the sum of such loss during reverse recovery and loss in the conducting state is attempted, a problem arises in that such a design is difficult to implement in a diode used for an inverter circuit, a chopper circuit or the like in which the period of each of the conducting and blocking states is not fixed.